As a technique for recording and/or reproducing digital data, there is a data recording technique employing an optical disc, inclusive of a magneto-optical disc, such as, for example, CD (Compact Disc), MD (Mini-Disc), or DVD (Digital Versatile Disc), as a recording medium. The optical disc is a generic term for a recording medium comprised of a disc of a thin metal sheet protected with plastics and which is irradiated with laser light. A signal is read out as changes in the light reflected from the disc.
The optical disc may be classified into a replay-only type, such as CD, CD-ROM or DVD-ROM, and a user-recordable type, such as MD, CD-R, CD-RW, DVD-R, DVD−RW, DVD+RW or DVD-RAM. Data recording on the user recordable type disc is enabled by exploiting a magneto-optical recording system, a phase change recording system or a dye film change recording system. The dye film change recording system, also termed a write-once recording system, allows for data recording only once and does not allow for rewriting, and hence may be used with advantage for data storage. On the other hand, the magneto-optical recording system or the phase change recording system allows for data rewriting and is utilized for a variety of fields of application including recording of various content data such as music, pictures, games or application programs.
Recently, a high density optical disc, termed DVR (Data and Video Recording) has been developed in an attempt to increase the data storage capacity appreciably.
For recording data on a recordable disc, such as a disc of the magneto-optical recording system, dye film change recording system or the phase change recording system, suitable guide means are necessitated for tracking to a data track. To this end, a groove is formed in advance as a pre-groove, with the groove or a land (an area of a trapezoidal cross-section defined between neighboring grooves or neighboring turns of the groove) being used as a data track.
It is also necessary to record the address information at a preset position on the data track such as to permit data to be recorded at a preset location on the data track. There are occasions where this address information is recorded by wobbling or meandering the groove.
Specifically, the sidewall section of the data recording track, formed in advance as a pre-groove, is wobbled in keeping with the address information.
By so doing, the address may be read out from the wobbling information, obtained as the reflected light information during recording and/or reproduction, such that data can be recorded and/or reproduced at a desired location without the necessity of pre-forming bit data etc. on the track for indicating the address.
By adding the address information as the wobbled groove, it is unnecessary to provide discrete address areas on the track to record the address as e.g., bit data, with the result that the recording capacity for real data can be increased in an amount corresponding to the address area which might otherwise have to be provided as described above.
Meanwhile, the absolute time (address) information, expressed by the wobbled groove, is termed the ATIP (Absolute Time in Pre-groove) or ADIP (Address in Pre-groove).
It should be noted that if, in the high density disc, recently developed, such as DVR, recording and/or reproduction of phase change marks is performed with a disc structure having a cover layer (substrate) of 0.1 mm along the direction of disc thickness, using a combination of the laser light with a wavelength of 405 nm, or so-called blue laser light and an objective lens with a NA of 0.85, 23.3 GB (giga-byte) of data can be recorded on a disc of 12 cm in diameter, for a data block of 64 kB (kilobyte) as a recording and/or reproducing unit, with the track pitch of 0.32 μm and a line density of 0.12 μm, with the format efficiency being approximately 82%.
If, with the similar format, the line density is set to 0.112 μm/bit, data with the capacity of 25 GB can be recorded and/or reproduced.
It is noted that there is raised a further drastically increased data capacity, so that it may be contemplated that the recording layer is of a multi-layer structure. For example, if the recording layer is of a double layer structure, the recording capacity may be 46.6 or 50 GB, or twice the above-mentioned capacity.
However, with the recording layer with a multi-layer structure, problems are raised as to desirable disc layout or as to achieving operational reliability.
There is also raised a problem as to achieving compatibility with the single layer optical disc.
It is also necessary to take into consideration the accessibility to the first and the following layers at the time of recording and/or reproduction.